1. Field of the Invention
The present invention relates to methods of making components for electrochemical cells, in particular, catalyst-coated membranes, gas diffusion electrodes, and membrane electrode assemblies.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant into electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly that includes a solid polymer electrolyte membrane disposed between two electrodes. The membrane electrode assembly is typically interposed between two electrically conductive flow field plates to form a fuel cell. These flow field plates act as current collectors, provide support for the electrodes, and provide passages for the reactants and products. Such flow field plates typically include fluid flow channels to direct the flow of the fuel and oxidant reactant fluids to an anode electrode and a cathode electrode of each of the membrane electrode assemblies, respectively, and to remove excess reactant fluids and reaction products. In operation, the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit. Typically, a number of fuel cells are electrically coupled in series to form a fuel cell stack having a desired power output.
The anode electrode and the cathode electrode each contain a layer of anode catalyst and cathode catalyst, respectively. The catalyst may be a metal, an alloy or a supported metal/alloy catalyst, for example, platinum supported on carbon black. The catalyst layer typically contains an ion conductive material, such as Nafion®, and, optionally, a binder, such as polytetrafluoroethylene. Each electrode further includes an electrically conductive porous substrate, such as carbon fiber paper or carbon cloth, for reactant distribution and/or mechanical support. The thickness of the porous substrate typically ranges from about 50 to about 250 microns. Optionally, the electrodes may include a porous sublayer disposed between the catalyst layer and the substrate. The sublayer usually contains electrically conductive particles, such as carbon particles, and, optionally, a water repellent material for modifying its properties, such as gas diffusion and water management.
One method of making membrane electrode assemblies includes applying a layer of catalyst to a porous substrate in the form of an ink or a slurry typically containing particulates and dissolved solids mixed in a suitable liquid carrier. The liquid is then removed to leave a layer of dispersed particulates, thereby forming an electrode. An ion-exchange membrane, such as a polymer electrolyte membrane, is then assembled with an anode electrode and a cathode electrode contacting opposite surfaces of the membrane such that the catalyst layers of the electrodes are interposed between the membrane and the respective substrate. The assembly is then bonded, typically under heat and pressure, to form a membrane electrode assembly. When sublayers are employed, the sublayer may be applied to the porous substrate prior to application of the catalyst. The substrate is commonly referred to as a gas diffusion layer or, in the case when a sublayer is employed, the combination of the substrate and sublayer may also be referred to as a gas diffusion layer.
Conventional methods of applying catalyst to gas diffusion layers to form gas diffusion electrodes include screen-printing and knife-coating. However, when applying low loadings of catalyst to a substrate to form a gas diffusion electrode, it is difficult to obtain a smooth, continuous catalyst layer (i.e., no discontinuities across the layer) with a uniform thickness due to the surface roughness of the substrate. This can result in fuel cell performance and/or durability being comprised.
Alternatively, a layer of catalyst can be applied onto both surfaces of the polymer electrolyte membrane to form a catalyst-coated membrane, and then assembled with porous substrates to form a membrane electrode assembly. For example, a catalyst slurry may be applied directly onto the membrane by microgravure coating, knife-coating, or spraying.
However, the use of a catalyst containing a catalytic material and a hydrophobic binder is desirable for fuel cell durability. As discussed in U.S. Pat. No. 6,517,962, fuel cells in series are potentially subject to voltage reversal, a situation in which a cell is forced to opposite polarity by the other cells in series. This can occur when a cell is unable to produce the current forced through it by the rest of the cells. Damage due to voltage reversal can be mitigated by increasing the amount of water available for electrolysis during reversal, thereby using the current forced through the cell in the more innoculous electrolysis of water rather than the detrimental oxidation of anode components. By restricting the passage of this water through the anode structure and into the exhaust fuel stream, more water remains in the vicinity of the catalyst. This can be accomplished, for example, by making the anode catalyst layer impede the flow of water (either in the vapor or the liquid phase). For instance, the addition of a hydrophobic material such as PTFE and/or FEP to these layers will make them more hydrophobic, thereby hindering the flow of water through the anode. However, if these polymers are not sintered, they may not be sufficiently hydrophobic and may wash out of the catalyst layer over time. Using conventional methods of applying the catalyst layer directly to the membrane, the catalyst layer would have to be sintered with the membrane. However, sintering temperatures are usually higher than the thermal degradation temperature of the ionomer. For example, Nafion® membranes typically start to decompose at about 250° C. Thus, if the membrane is coated with the catalyst having a hydrophobic binder and then subjected to temperatures sufficient to sinter the hydrophobic binder (e.g., 330° C. for PTFE), the ion-conducting and water uptake properties of the ionomer may be decreased or destroyed.
Accordingly, while advances have been made in this field, there remains a need for improved methods of making gas diffusion electrodes and catalyst-coated membranes. The present invention addresses this issue and provides further related advantages.